In Reed and Mayer’s superior ninth mystery set in sixth-century Constantinople (after 2010’s Eight for Eternity), Justinian sets John, the emperor’s lord chamberlain, an almost impossible, high-stakes task. John must discover who killed Justinian’s wife, Empress Theodora, who has died after a long and painful illness. John is skeptical that Theodora was poisoned, especially since access to the empress was strictly limited, but he fears his family may suffer if he fails to please the capricious dictator, rumored to be a demon in human form. Since many people had reason to want Theodora dead, John has to work his way through every reasonable possibility while dreading the end of Justinian’s patience. The puzzle is challenging enough to keep readers searching for clues, but the triumph of the authors lies in their spot-on recreation of the political and bureaucratic climate of the times. (Mar.)